Okay, I get it that new players sometimes don't know about the enmity skills even though the Training quest chain should have taught them, that's why I noted that the players doing this are not at the base level for the instance, they're at the sync level so likely this is not their first or second dungeon.

Also, I agree that the dropping of buffs when synced means you can easily forget to re-enable them at the start of a dungeon, I've done that myself at times, but that doesn't explain why the tank in the run my friend left turned it off after turning it on when prodded.

Thanks for the comments, I posted this in case there was a good reason why a tank who after turning this skill on would turn it off, or at times refusing to do so after being asked more than once, would turn it off; the answer seems to be the obvious 'no'.